Material coatings are pervasive in our energy-intensive, consumption-oriented society. Coatings provide: immunity to corrosion, thermal insulation, shielding, as well as appearance enhancement, and an aid in identification.
During the life of many manufactured products, such as aircraft and ships, painted coatings require removal and replacement for a variety of reasons. For example, refurbishment of the paint on aircraft is a regular maintenance item. Commercial airlines repaint their aircraft about every 4-5 years of service, depending on the age and operating hours of the craft. The United States military typically repaints its aircraft after three years of service, or less.
The removal of paint from the surfaces of aircraft presents special problems, in part, due to their large and irregularly shaped surfaces. Another difficulty is that, because the surfaces of aircraft are principally lightweight aluminum or organically based composite materials, they are particularly susceptible to damage which could compromise their structural integrity.
Many different methods have been used to remove painted coatings from aircraft. One type, the "particle medium blast" (PMB) method involves impinging the surface to be stripped with particles such as BB's, steel shot, wheat, starch, and/or sand. However, this method generates unwanted dust, requires large quantities of bulk blast materials and is noisy. PMB is also difficult to control. If the impinging particles dwell too long at one location, the surface of the aircraft may become pitted or stress hardened which can change the loading on that portion of the aircraft. PMB also damages putty joints often found on aircraft between surface plates. Another major problem is when paint materials, which are generally toxic, are impinged by the blast particles, the waste produced is toxic. Toxic waste requires special handling in order to dispose of it in a manner which minimizes damage to the environment. Another problem with PMB is that it generates a lot of dust which obscures the area being stripped. This impairs visual feedback necessary to control the process, resulting in damaged surfaces.
Some airlines have used water jets to remove paint from aircraft. However, friction caused as a water jet impacts a surface such as aluminum generates heat which can damage the aluminum, especially if it is thin, like that found on aircraft. Another problem with this method is that the high pressure water can penetrate into the internal regions of the aircraft which are susceptible to water damage.
It is also known in the art to apply chemical compounds to painted surfaces in order to chemically breakdown the layers of paint, thereby stripping the paint away from the surface to be exposed. Certain of such chemical compounds have been used to remove paint from aircraft. However, such compounds may pose a risk to human health, are usually toxic, and often not biodegradable. Overall, these types of compounds are difficult and costly to dispose of because they present serious environmental problems.
Mechanical paint removal techniques have also been employed. For example, U.S. Pat. No. 4,836,858, entitled "Ultrasonic Assisted Paint Removal Method" discloses a hand held tool which uses an ultrasonic reciprocating edge placed in contact with the surface to be stripped. Unfortunately, employment of this tool is labor intensive and relies upon the skill of a human operator to use it effectively. Further, control of this tool is a problem because the aircraft surface may still be damaged if there is excessive tool dwell at one location.
Radiant energy paint removal techniques are also known in the art. One such system uses a laser and video frame grabber in a video controlled paint removal system in which paint is stripped from a surface using particle medium blast methods of the type discussed above while a video camera converts images of the surface being stripped into electronic data signals. The data signals are used to control the particle blast. A processor compares the data signals with parameters stored in a memory to determine whether sufficient paint has been removed from the surface being stripped. If an insufficient amount of paint has been removed, then the surface continues being irradiated. If the optically irradiated area has been adequately stripped, the processor directs the particles to strip another area.
A significant problem with the video controlled paint removal system is that the amount of data which is generated and which must be processed is enormous. Hence, real time control of video controlled paint removal systems is extremely difficult. Another problem with a video controlled paint removal system is that large amounts of dust are generated from the effect of the particle blast impacting the surface being stripped. The dust can impair the scene being observed by the video camera, also making real time control of the process difficult.
Thus, it can be appreciated that coating removal, and particularly, the removal of paint from large, delicate surfaces such as found on aircraft is a problem which has not yet been satisfactorily solved.
The practice of photoacoustic spectroscopy (PAS) for analyzing a given solid material is also known. In PAS, light energy is absorbed by a solid material, converted into an acoustic wave or pressure pulse which is characteristic of the solid material, and then converted into an electrical signal for analysis purposes. In such PAS systems, a laser is employed to direct light energy at the solid material, although other types of light sources may also be used. The material absorbs the light energy in a way characteristic of the particular solid material being irradiated. Any light absorbed by the material is converted, in part or in whole, to heat. An acoustic signal results from the time dependent heat flow from the solid material to the surrounding gas. The heat flow causes oscillatory time dependent pressure in a small volume of gas at the solid-gas interface. An additional source of time dependent pressure in the gas can arise when the absorbing solid ablates and subsequently burns to release its heat of combustion. It is this motion of the gas which produces the acoustic signal that is characteristic of the solid, also referred to as the photoacoustic characteristic of the solid.
The strength of the photoacoustic wave is approximated by a theory attributable to Taylor and Sedov (D. A. Freiwald and R. A. Axford, J. Appl. Phys., Vol. 46, p. 1171, 1975), which predicts the pressure behind the shock for a spherical blast wave to be: EQU P=[2/(.gamma.+1)](4/25)(.xi..sub.o.sup.5)(E/R.sup.3)
where .gamma. is the heat capacity ratio for the ambient air, .xi. is a constant, (.xi.=1.03 for air), E is the energy absorbed by the surface of the material being irradiated, and R is the distance of the pressure wave from the surface. The Taylor, et al. theory predicts that the strength of the wave is directly proportional to the energy absorbed by the surface, which in turn depends on the absorptivity of the surface at the wavelength of the light source.
An example of a system which detects and measures a photoacoustically generated pressure pulse to control a material removal process is described in U.S. Pat. No. 4,504,727, "LASER DRILLING SYSTEM UTILIZING PHOTOACOUSTIC FEEDBACK." The system described in the '727 patent uses photoacoustic feedback to control laser drilling of a multilayered printed circuit board. The system analyzes the photoacoustic feedback signals by comparing the photoacoustic outputs from the different layers of the circuit board with reference signals stored in a memory, and adjusts the laser parameters such as pulse duration, wavelength, and energy output, pulse repetition rate, and the number of pulses for each successive layer accordingly. The circuit board is mounted to an X-Y moving table under the direction of the control system which positions selected hole sites on the circuit board under the laser beam. However, the system described in the '727 patent is not suitable for removing selected layers of material from large surfaces in predetermined patterns.
The types of lasers described in the '727 patent are very long wavelength devices, therefore, every surface looks black to such lasers, with little or no reflected light energy. Because of this, it would be difficult to distinguish material coatings based on their albedo. Therefore, the use of a far infrared optical energy source to ablate material from a structure likely results in generation of photoacoustic pulses that depend more on the mechanical damping properties and resonance of the structure than on the characteristics of the ablating material.
Furthermore, the system described in the '727 patent modulates the output of the light source as a function of the amplitude of the photoacoustic signals. Such modulation disadvantageously shortens the useful life of the laser.
Thus, there is great need for a system and method which can easily and inexpensively remove coatings and which does not present the environmental problems of some of the systems and methods described above. A need also exists for a coating removal system and method that can be controlled to avoid subjecting a surface which is to be exposed to an excessive amount of energy which would damage delicate structures. A further need exists for a coating removal systems which can be automated. Still further, a need exists for a system and method which promotes a long service life of the light source.